libstdc++: Fix doxygen generation to work with relative paths

libstdc++: Fix doxygen generation to work with relative paths

In r12-826 I tried to remove some redundant steps from the doxygen
build, but they are needed when configure is run as a relative path. The
use of pwd is to resolve the relative path to an absolute one.

libstdc++-v3/ChangeLog:

	* doc/Makefile.am (stamp-html-doxygen, stamp-html-doxygen)
	(stamp-latex-doxygen, stamp-man-doxygen): Fix recipes for
	relative ${top_srcdir}.
	* doc/Makefile.in: Regenerate.